This invention is directed to a hydraulic driving hydraulic system for a vehicle, and in particular to a system which has an adjustable pump and at least one motor, with the system being held at a load pressure level. The hydraulic motor is shortened in overall dimension by rearrangement of the elements of the motor to eliminate gaps occurring previously. The invention is further directed to a braking device for the system, having at least one braking cylinder with a spring chamber and a release chamber, and a braking valve. In the braking position, the braking valve supplies the spring chamber with load pressure and the release chamber with tank pressure, and in the release position, the braking valve supplies the spring chamber with tank pressure and the release chamber with load pressure.
A driving system of this kind is, for example, known from EP 0 909 690 A2. It is particularly suited for agricultural and industrial working machines, like farm tractors, harvesters and construction machines, and is most frequently used in skid steered loaders. In the known case, there is an auxiliary pump, which supplies a number of working motors, on whose outlet two pressure limiting valves ensure that a load pressure is produced, which is supplied to both the hydraulic system and the braking device. If the hydraulic system is not activated, only the spring acts to brake the vehicle, for example to prevent it from rolling down a slope. If, however, the hydraulic system is activated, the braking valve is operated; causing that the load pressure in the release chamber makes the spring in the spring chamber inactive. When, however, an error requiring braking occurs, the braking valve is de-energised, so that the spring, supported by the load pressure in the spring chamber, causes a safe braking, also in relation to a rotation of the motor. One disadvantage is that in the working system a relatively large oil quantity has to be brought to a higher pressure.
A commercially available hydraulic driving device, shown in part in FIG. 1, is arranged so that a gap occurs between a friction-disc pack used as part of the braking device and a nut for retaining the output shaft bearing in place. The gap increases the overall length of the motor. In addition, the motor using helical springs and an elongated annular piston, which also contribute to an unnecessarily overall axial length of the motor.
The invention is based on the task of providing a hydraulic driving system, which has a new braking device and a motor of reduced overall axial length.
The motor has a flushing line, which is connected with the spring chamber and led to a tank via the braking valve. In the release position the braking valve opens the flushing line in the direction of the tank, and in the braking position prevents a pressure reduction in the flushing line.
With this arrangement, the lines of the flushing system and the braking device are partly combined, which causes savings and new line arrangements. This combination is possible, as the flushing operation is not disturbed, as during release operation the flushing line is connected with the tank and during braking operation flushing is not required. At least partly, the pressure build-up in the spring chamber is supported by the pressure medium flowing in through the flushing line.
In a one embodiment, in the braking position the braking valve blocks the flushing line. This blocking prevents a pressure medium discharge via the flushing line and the leakage line of the motor. A pressure will automatically build up in the spring chamber, which is almost equal to the load pressure of the hydraulic system.
An equally favorable alternative is that in the braking position the braking valve supplies the flushing line with the load pressure. Here, the pressure increase in the spring chamber, caused by the load pressure, is supported by the supply of pressure medium from the flushing line.
As a further alternative, the leakage line of the motor is connected to the release chamber and the flushing line is connected to the spring chamber. In yet a further embodiment, the leakage line is connected to the spring chamber and the flushing line is connected to the release chamber. In either alternative, an increase of the braking force occurs in the braking position when the braking valve blocks connection of the spring chamber to tank pressure, causing a pressure buildup in the spring chamber.
It is advantageous that a load pump is provided to maintain the load pressure in the system, at the outlet of which pump a pressure control valve is connected. The load pump can be dimensioned for relatively low pressures and small filling quantities. The size of the load pressure only has to be chosen so high that the braking device is certain to work.
To maintain the load pressure in the motor a flushing valve is provided, whose outlet is also connected with the spring chamber via a throttle. Thus, also the outlet line of the flushing valve can be combined with lines of the braking device.
In the hydraulic motor, to shorter the length, the shoulder is arranged on the side of the bearing arrangement opposite from the end of the output side of the output shaft, and the friction-disc pack bears on the bearing arrangement.
With this solution, a gap between the friction-disc pack and the bearing arrangement is avoided. Additionally, the output shaft can be correspondingly shortened at the side of the hydraulic motor.
Preferably, the spring arrangement is a cup spring. This gives an additional shortening of the overall axial length.
Thus, the annular piston can be made as a substantially flat annular disc. This also causes a reduction of the overall length.